Arsenic and arsenic containing compositions have been widely used in Australia as sheep and cattle dips, and have also been used in pesticides. Mercury and mercury containing compositions have also been widely used in Australia and other countries. With the phasing out of arsenic and mercury compounds (due to their toxicity), a large stockpile of waste arsenic and mercury components exists.
Organo nickel and chromium, and nickel and chromium containing compositions are widely used in Australia and other countries. Metal plating and anodising processes use these compounds, and the processes generate waste nickel and chromium residues which are quite concentrated and are stored in drums. The residue is hazardous and toxic and a large stockpile of waste nickel and chromium components exists.
A variety of organic waste materials, some of which are hazardous, are produced as by-products of various industrial processes. These organic wastes include contaminated oils and greases, organo-chlorine compounds including pesticides, and chlorinated aromatic compounds including chlorobenzene and polychlorinated biphenyl compounds.
The above mentioned waste materials and toxic components are housed in drums that have a limited life. The components in the drum are typically in the form of a contaminated liquid or sludge that is almost impossible to safely encapsulate. The sludge contains a variety of contaminants such as rust from the drum, waste particles, solids, and a variety of liquids.
Radioactive materials and components are also hazardous substances and it is difficult to find an acceptable method of their disposal. Various proposals have been trialled for encasement or encapsulation of radioactive materials which can then be disposed of by burial in uninhabited areas.
Attempts to encapsulate hazardous waste materials in concrete have met with limited success as concrete and cement do not bind well in the presence of such contaminants. However, concrete or concrete-like products would be otherwise ideal for encasement as the concrete is hard, has a very long life, and can be moulded prior to setting.
U.S. Pat. No. 6,200,381 discloses a settable binder composition consisting essentially of a mixture of a calcium carbonate and a caustic magnesium oxide. The mixture may be derived from a naturally-occurring dolomite which has been calcined to cause preferential decarbonisation of the magnesium carbonate by liberating carbon dioxide, without substantially decarbonising the calcium carbonate. This calcination results in a mixture of magnesium oxide, magnesium carbonate and calcium carbonate (MgO, MgCO3 and CaCO3). Alternatively, a synthetic blend may be formed by mixing calcium carbonate with preformed caustic magnesium oxide. Various additives and fillers may be included in the binder to modify the properties of the set composition. The resulting product was found to have high compressive strength and other properties which make it ideal for manufacturing building products such as bricks, blocks, pavers, tiles, etc.
WO98/54107 describes a method of encapsulating hazardous waste materials using essentially the same settable composition containing calcium carbonate and a caustic magnesium oxide. Tests conducted on the encapsulated material indicate that virtually none of the hazardous waste material was leached out of the set composition, which has a concrete-like appearance.
The presence of the calcium carbonate in these settable binder compositions:    (i) helps to control the setting rate, ie., the higher the amount of calcium carbonate the slower the setting rate. The control of setting rate is important in the manufacture of building materials, where a “working time”, or time during which the set is slower, is required in order to allow placement into moulds.    (ii) modifies certain physical properties of the set composition, including lowering the shrinkage and brittleness of the final product.
In some situations successful waste encapsulation is not reliant on having high, unconfined compressive strength, nor is low shrinkage or increased working time to allow placement in moulds required. Working time can be controlled sufficiently by adding extra water when forming a slurry from the mixture of the hazardous waste material and the binder composition. While the absence of the calcium carbonate is accompanied by a significant reduction in the unconfined compressive strength of the set composition, disposal regulations relating to the treatment of waste do not require a high strength product containing encapsulated waste materials.